Infrared ear thermometer

ABSTRACT

An infrared ear thermometer includes a detector head housing, a heat sink, a recess formed in the heat sink, a thermopile sensor mounted within the recess, a thermistor, and temperature determination circuitry. The recess defines an aperture that limits the field of view of the thermopile sensor. The thermal capacities and conductivities of the heat sink and the thermopile sensor are selected so that the output signal of the thermopile sensor stabilizes during a temperature measurement. A method of determining temperature using the ear thermometer takes successive measurements, stores the measurements in a moving time window, averages the measurements in the moving window, determines whether the average has stabilized, and outputs an average temperature. A method of calculating a subject&#39;s temperature determines the temperature of a cold junction of the thermopile, looks up a bias and slope of the thermopile based upon the temperature of the cold junction, measures the output of the thermopile, and calculates the subject&#39;s temperature based upon a linear relationship between the output and the subject&#39;s temperature. The linear relationship is defined by the bias and the slope.

RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 09/395,797,now U.S. Pat. No. 6,435,711, filed Sep. 14, 1999, which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to temperature measurement and, moreparticularly, the invention relates to infrared clinical thermometers.

2. Description of the Related Art

Conventional ear thermometers employ an infrared (IR) detector forsensing the temperature inside the ear at the tympanic membrane. Theinfrared detector is mounted within a heat sink so as to stabilize anambient reference temperature. As conventionally mounted, the detectoris too large to be inserted into the ear canal leading to the tympanicmembrane. Accordingly, a waveguide, typically formed of a polished tube,is interposed between the tympanic membrane and the IR detector. The useof a waveguide involves a number of drawbacks resulting, for example,from its non-zero emissivity, its relatively high cost, and thecomplexity of the resultant structure.

SUMMARY OF THE INVENTION

One embodiment of the invention is an infrared ear thermometer. Theinfrared ear thermometer-includes a detector head housing having a heatsink with a recess. A thermopile sensor is mounted in the recess. Therecess defines an aperture that limits the field of view of thethermopile sensor. The thermometer also includes a thermal sensor andtemperature determination circuitry configured to calculate atemperature in response to output of the thermopile sensor and thethermal sensor.

Another embodiment of the infrared ear thermometer includes a detectorhead housing having a heat sink with a recess. A thermopile sensor ismounted in the recess. The thermopile sensor has a hot junction and acold junction, the hot junction being responsive to infrared radiation.An output signal of the thermopile sensor is related to a temperaturedifference between the hot junction and the cold junction. A thermalcapacity of the hot junction, a thermal conductivity between the hotjunction and the cold junction, a thermal capacity of the cold junction,a thermal conductivity between the cold junction and the heat sink, anda thermal capacity of the heat sink are selected so that the outputsignal of the thermopile sensor stabilizes or has a flat peak during atemperature measurement. A thermistor is mounted in thermalcommunication with the cold junction. Temperature determinationcircuitry calculates an output temperature in response to the outputsignal of the thermopile sensor and an output of the thermistor.

Another embodiment of the infrared ear thermometer includes a detectorhead housing having a heat sink with a recess. A thermopile sensor ismounted in the recess. The thermopile sensor has a hot junction and acold junction, the hot junction being responsive to infrared radiation.The output signal of the thermopile sensor is related to the temperaturedifference between the hot junction and the cold junction. A thermistoris mounted in thermal communication with the cold junction. A heat pipe,made of a thermally conductive material, surrounds a portion of thethermopile sensor, whereby the heat pipe prevents heat transfer from anear canal to the hot junction. Temperature determination circuitrycalculates an output temperature in response to the output signal of thethermopile sensor and an output of the thermistor.

Another embodiment of the invention is a method for determining thetemperature of a subject. Successive temperature measurements of thesubject are taken using an infrared thermometer. A plurality of mostrecent measurements are stored in a moving time window. An average of upto all of the plurality of stored measurements is successivelycalculated. The difference between successive averages is calculated anda determination is made whether the difference is less than apredetermined value. Finally, a calculated temperature is output.

Another embodiment of the invention is a method for calculating asubject's temperature based upon output values from a thermopile and athermistor of an infrared thermometer, wherein the thermistor is inthermal communication with a cold junction of the thermopile, andwherein the hot junction is in infrared communication with an object tobe measured. The temperature of the cold junction is determined with thethermistor. A bias and a slope of the thermopile are looked up in alookup table based upon the temperature of the cold junction. The outputsignal of the thermopile is measured. Finally, a temperature iscalculated based upon a linear relationship defining temperature as afunction of the output signal of the thermopile in terms of the bias andthe slope.

Another embodiment of the invention is an infrared thermometer formeasuring temperature of a subject's forehead. The infrared earthermometer includes a detector head having a metal heat sink. Athermopile sensor is mounted to the front of the heat sink with athermopile sensor mounted in the recess. A thermistor is mounted inthermal communication with a cold junction of the thermopile sensor. Theouter surface of the detector head includes an extension for placementagainst the subject's forehead, wherein the extension forms an airpocket isolating a portion of the forehead from air flow outside the airpocket.

Another embodiment of the infrared thermometer for measuring temperatureof a subject's forehead includes a detector head having a metal heatsink. A thermopile sensor is mounted to the front of the heat sink witha thermopile sensor mounted in the recess. A first temperature sensormeasures a cold junction temperature of the thermopile sensor. A secondtemperature sensor provides fast measurement of a room ambienttemperature. An electronic circuit calculates the temperature of thesubject based in part upon the input of the second temperature sensor.

Another embodiment of the invention is a detector head housing for aninfrared clinical thermometer. The housing includes a body made of asoft plastic material. The body has an outer surface configured to makecontact with a subject. A liquid having a high thermal capacity fills aportion of a cavity formed within the body. The high thermal capacityliquid thermally isolates an infrared sensor within the body from heatgenerated by the subject.

Another embodiment of the infrared ear thermometer includes a detectorhead housing having a heat sink with a recess. A thermopile sensor ismounted in the recess. A sleeve that defines an aperture that limits thefield of view of the thermopile sensor fits into the recess. Thethermometer also includes a thermal sensor and temperature determinationcircuitry configured to calculate a temperature in response to output ofthe thermopile sensor and the thermal sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred and other embodiments of the present invention aredescribed below in connection with the drawings in which like referencenumbers represent corresponding components throughout, and in which:

FIG. 1 illustrates an ear thermometer constructed and operative inaccordance with a preferred embodiment of the present invention;

FIG. 2 illustrates a cross section of a detector head in accordance witha preferred embodiment of the ear thermometer;

FIG. 3 illustrates a detector head constructed in accordance with analternative embodiment of the present invention;

FIG. 4 is a graph of the possible output signals of a thermopiledetector as a function of time;

FIG. 5A illustrates a family of plots of the output of the thermopiledetector as a function of the temperature of the object being measured;

FIG. 5B illustrates a preferred method that can be used in accordancewith the present invention to calculate the temperature of an object;

FIG. 6 illustrates a preferred method for measuring temperature inaccordance with the present invention;

FIG. 7 illustrates a detector head adapted for measuring foreheadtemperature in accordance with an alternative embodiment to the presentinvention;

FIG. 7A schematically illustrates an infrared thermometer having asecond temperature sensor in accordance with embodiments of the presentinvention;

FIG. 8 illustrates an alternative embodiment of the detector headconfigured to function in conjunction with a variety of ear canal sizes;and

FIG. 9 illustrates an alternative embodiment of the detector headconfigured to provide improved heat transfer.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following description, reference is made to the accompanyingdrawings, which form a part hereof, and which show, by way ofillustration, specific embodiments in which the invention may bepracticed. It is to be understood that other embodiments may be utilizedand structural changes may be made without departing from the scope ofthe present invention. Where possible, the same reference numbers willbe used throughout the drawings to refer to the same or like components.Numerous-specific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be understoodby one skilled in the art that the present invention may be practicedwithout the specific details or with certain alternative equivalentdevices and methods to those described herein. In other instances,well-known methods, procedures, components, and devices have not beendescribed in detail so as not to unnecessarily obscure aspects of thepresent invention.

FIG. 1 illustrates an ear thermometer constructed and operative inaccordance with a preferred embodiment of the present invention. The earthermometer comprises a hand held housing 10, in which is disposed adetector head 12, temperature determining circuitry 14, which receivesinputs from the detector head 12, and a display 15 for displaying avisible indication of measured temperature. An electrical power source16 and optional audio output device 18 are also provided.

FIG. 2 illustrates a cross section of the detector head 12. The detectorhead 12 includes a tapered, generally conical, concavely curved outersurface 20 preferably formed of a soft thermal insulator such as foamedpolyurethane. The detector head 12 is configured to engage the ear canalof a subject. The outer surface 20 is preferably somewhat flexible, witha hardness equal to or less than Shore 40, which provides a pleasingtouch to a human ear. Disposed in the interior of the outer surface 20is a thermal insulative filler 22, preferably formed of a plasticmaterial, which defines a longitudinal bore 24 extending from a narrowfront (distal) end 26 to a rearward (proximal) end 28 of the detectorhead 12.

Disposed in the bore 24 is a heat sink 30 preferably comprising a highthermal conductivity, high thermal capacity rod. The heat sink 30 ispreferably made of copper. The heat sink 30 is preferably dimensioned tofit tightly in the bore 24. In one embodiment, the heat sink terminatesin a threaded end 19 with which the detector head 12 is attached to thehousing 10. A recess 32 in the heat sink 30 is preferably locatedadjacent the front end 26. An infrared (IR) transmissive window 34,typically formed of silicon, is preferably hermetically sealed at thefront end 26, thus sealing the bore 24. The IR transmissive window 34 ispreferably mounted with a thermal adhesive directly onto the heat sink30 such that the window 34 is in thermal connection with the heat sink30.

An IR sensor 40 is mounted directly onto a forward facing surface 42 ofthe heat sink 30 in recess 32 and is positioned so that the recesslimits the field of view of the sensor 40 to window 34. The IR sensor 40thus “sees” the heat sink 30 and the outside, as defined by the field ofview, but nothing else. The IR sensor 40 is preferably provided withouta conventional housing and is preferably mounted directly on the heatsink 30. In the preferred embodiment, the heat sink 30 serves as thehousing for the IR sensor 40. The IR sensor 40 is preferably athermopile sensor.

A thermistor 44 is preferably mounted inside the heat sink 30 at alocation adjacent that of surface 42. Electrical output signals fromboth the IR sensor 40 and the thermistor 44 are preferably provided topreamplification electronics 46, which can be mounted on the heat sink30. The output of the preamplification electronics 46 is provided to thetemperature determining circuitry 14 (FIG. 1) via a connecting socket48. In accordance with one embodiment of the invention, the temperaturedetermining circuitry 14 comprises a lookup table that maps actualmeasured values of the electrical output signals to temperatures,thereby providing a high degree of accuracy.

When the thermometer is inserted into a subject's ear canal, the IRsensor 40 receives IR radiation directly from the ear, with the onlyintermediary being the window 34. When properly positioned in the earcanal, the IR sensor 40 is preferably located adjacent the tympanicmembrane of the subject, at a distance of approximately 2.5 cm.Preferably, no waveguide is employed.

FIG. 3 illustrates a detector head 50 constructed in accordance with analternative embodiment of the present invention. The detector head 50has a general conical shape with a smaller diameter at the front(distal) end 56 and a larger diameter at the rearward (proximal) end 58.A conical heat sink 51, preferably made of metal, is positioned withinthe detector head 50. A thermopile detector 60 is mounted on the heatsink 51 near the front end 56.

The thermopile detector 60 is preferably a regular thermopile detector,such as TPS333, made by Heiman, Germany. The thermopile detector 60preferably comprises a base 62 made of stainless steal, a housing 64also made of stainless steal, a silicon window 66, a thermopile sensor68, and a thermistor 61. The thermistor 61 and a cold junction 67 of thethermopile sensor 68 are thermally connected to the base. A hot junction65 is mounted on a thin membrane such that the thermal conductivity tothe cold junction 67 and the base 62 is made as low as possible. Radiantheat striking the hot junction 65 causes the thermopile to produce anoutput signal directly proportional to the incident radiation. The hotjunction temperature is referenced to the cold junction temperature. Thecold junction temperature is preferably measured by the thermistor 61.

A sleeve 75 is mounted to the detector housing 64 or to the heat sink 51or both. The sleeve 75 defines an aperture 70 that limits the field ofview 78 of the thermopile sensor 68. The sleeve 75 preferably is made ofa metal with a high thermal conductivity, such as copper, and preferablyhas a polished surface coated with gold. The sleeve 75 and the aperture70 allow the sensor 68 to receive radiation only from the object to bemeasured. The sleeve 75 and the aperture 70 also prevent reflectionsfrom the sleeve 75 and radiation emitted from the outer parts of thedetector head 50 from reaching the hot junction 65 surface. The sleeve75 is preferably also in contact with the detector housing 64.

A metal heat pipe 54, preferably made of copper coated with nickel orgold, is mounted around the heat sink 51, the detector 60, and thesleeve 75. The metal heat pipe 54 transfers heat from the front end 56of the head 50 to the heat sink 51, to the thermistor 61, and to thecold junction 67. Accordingly, the heating of the hot junction 65results from infrared radiation as opposed to heat transfer from the earcanal while the head 50 is inserted into the ear. Furthermore, an airgap 55 between the sleeve 75 and the heat pipe 54 isolates the sleeve75, the window 66, and the detector housing 64 from heat transfer fromthe ear canal. An external layer of foamed plastic material 52, such asfoamed polyurethane, covers the heat pipe 54. The foamed layer 52 has avery low heat conductivity and therefore isolates the detector head 50from heat transfer from the ear canal. The foamed layer 52 also providesa soft touch in the ear canal to prevent irritation and make themeasurement more comfortable.

FIG. 4 illustrates plots of the possible output signals of thethermopile detector 60 as a function of time. The output signal of thethermopile detector is proportional to the temperature differencebetween the hot junction 65 and the cold junction 67. The hot junction65 is heated by the infrared radiation with a short time constant ofabout 0.2 seconds. The thermal conductivity between the hot junction 65and the cold junction 67, the thermal conductivity between the coldjunction 67 and the heat sink 51, and the thermal capacity of the heatsink 51 are preferably selected to produce an output signal thatstabilizes as shown by a curve 81. Another acceptable configurationproduces a curve 82 having a relatively flat peak. A slowly increasingsignal with a continuous positive slope as shown by a curve 80 is lessdesirable.

FIG. 5A illustrates a family of plots of the signal from the thermopiledetector 60 as a function of the temperature of the object beingmeasured. Each of the family of lines depicts the relationship betweenthe signal and the measured temperature at a certain ambient temperatureof the cold junction 67. For each ambient temperature, therepresentative line can be described by a bias and a slope asillustrated in FIG. 5A. The temperature determining circuitry 14preferably stores the bias and the slope of each line in lookup tables.Interpolation can be used to determine the bias and slope fortemperatures between those stored in the lookup tables. For certainthermopile detectors, the family of lines may be even be degeneratedinto one line.

FIG. 5B illustrates a preferred method that can be used in accordancewith the present invention to calculate the temperature of an objectusing the measured temperature of the cold junction 67 and the signalfrom the thermopile detector 60. At a first step 102, the temperaturedetermining circuitry 14 looks up, in the lookup tables, the bias andslope corresponding to the two temperatures closest to the cold junctiontemperature. At a step 104, the temperature determining circuitry 14uses interpolation to determine the bias and slope of a linecorresponding to the cold junction temperature. The line defines thelinear relationship between object temperature and the signal from thethermopile detector 60 at the cold junction temperature. At a step 106,the temperature determining circuitry 14 uses the linear relationshipdefined by the interpolated bias and slope to calculate the objecttemperature based upon the measured thermopile detector signal.

FIG. 6 illustrates a preferred method for measuring temperature inaccordance with the present invention. At a first step 122, thethermometer is inserted into the ear. At a next step 124, thetemperature determining circuitry 14 waits until the measuredtemperature has exceeded a certain threshold, such as 34 C, beforeproceeding onto a next step 126. This determination can be made withoutperforming the steps of FIG. 5B by generating an approximate conversionof the signal from the thermopile detector 60 to temperature.Alternatively, the determination can be based upon an expectedapproximate thermopile output at the threshold temperature. At the nextstep 126, the circuitry 14 imposes a time delay before beginning an eartemperature measurement in order to allow the user enough time to fullyinsert the thermometer into the ear canal. At a next step 128, thetemperature determining circuitry 14 begins taking temperaturemeasurements (i.e., measurements of the output signal from thethermopile detector 60) in a moving measurement window, such as a spanof 0.5 seconds. At a next step 130, the temperature determiningcircuitry 14 moves the measurement window in time and calculates anaverage temperature (i.e., an average value for the output signal fromthe thermopile 60) from the measurements in the measurement window. Inone embodiment, the temperature determining circuitry 14 may discardpeak, high, and/or low readings from the measurement window incalculating the average value of the thermopile signal. At a next step132, the temperature determining circuitry 14 determines whether asteady state value of the thermopile output signal has been reached. Asteady state is preferably reached when the change in the average valuewithin the moving window is less than a predetermined amount. If asteady state value has not been reached at the step 132, control ispassed back to the step 130. Otherwise, if a steady state value has beenreached at the step 132, the steady state temperature is calculated anddisplayed at a last step 134 by performing the steps in FIG. 5B toconvert the measured thermopile output signal to temperature.

In an alternative embodiment of the method of FIG. 6, the temperaturedetermining circuitry 14 successively stores, instead of a thermopileoutput value, a calculated temperature in the moving time window. Themethod proceeds as described above until the calculated averagetemperature has reached a steady state.

FIG. 7 illustrates an embodiment of the detector head 50 adapted formeasuring forehead temperature in accordance with an alternativeembodiment of the present invention. An extension 64 to the outersurface 52 provides for placement of the thermometer against a subject'sforehead 80. The extension 64 preferably forms an air pocket 65 toisolate a portion of the forehead 80 from air flow outside of the airpocket. An ambient room temperature may be determined by measuring,through the thermistor 61, the temperature of bracket 62. The ambientroom temperature may them be taken into account, using known techniques,preferably using a lookup table, in order to determine the temperatureof the subject based upon the measured temperature of the subject'sforehead. The embodiment of FIG. 7 is preferably stabilized in the roomtemperature where the measurement is to take place for at least twentyminutes due to the thermal mass of the bracket.

An additional embodiment, schematically illustrated by FIG. 7A, providesa second thermistor 63 with fast response for measuring ambient roomtemperature. Corrections can also be added to the lookup table toaccount for any immediate change in room temperature. By adding a secondthermistor 63 with a small thermal mass and good thermal contact to thesurrounding air, the required stabilization time may be shortened toless than one minute.

FIG. 8 illustrates an alternative embodiment of the detector head 50configured to function in conjunction with a variety of ear canal sizes.The detector head 50 generally has two main outer diameters. Thediameter of the front part 91 is smaller and can be inserted into theear canal to fix the thermometer direction. The rear part 92 has a muchlarger diameter, which always stops the detector head 50 at the entranceof the ear canal at a fixed distance from the tympanic membrane. Thisconstruction, which keeps the thermometer always at a fixed position inthe ear canal, solves one of the main causes of instability in takingtemperature measurements from the ear. The front part 91 of the detectorhead 50 is preferably made of foamed soft plastic material, such asfoamed polyurethane, and the rear part 92 is preferably made of aconventional plastic material, such as, for example, ABS.

FIG. 9 illustrates an alternative embodiment of the detector head 50configured to provide improved heat transfer. A soft plastic case 53filled with cooling liquid 57 is mounted around the heat sink 51, thedetector 60, and the sleeve 75. The liquid may be any liquid with a highthermal capacity, such as water. The liquid transfers heat from thefront end of the detector head 50 to the heat sink 51 and preventsheating of the sleeve 75 by heat transfer. The pressure of the liquid inthe soft plastic case provides a comfortable and soft touch wheninserted into the ear canal.

While certain exemplary preferred embodiments have been described andshown in the accompanying drawings, it is to be understood that suchembodiments are merely illustrative of and not restrictive on the broadinvention. Further, it is to be understood that this invention shall notbe limited to the specific construction and arrangements shown anddescribed since various modifications or changes may occur to those ofordinary skill in the art without departing from the spirit and scope ofthe invention as claimed. It is intended that the scope of the inventionbe limited not by this detailed description but by the claims appendedhereto.

What is claimed is:
 1. A method for determining the temperature of asubject, the method comprising: taking successive temperaturemeasurements of the subject using an infrared thermometer; storing aplurality of most recent temperature measurements in a moving timewindow; successively calculating an average of up to all of theplurality of stored measurements; calculating the difference betweensuccessive averages; determining whether the difference is less than apredetermined value; and outputting a calculated average temperature. 2.The method of claim 1, wherein the infrared thermometer is configured tomeasure a temperature of a subject's forehead.
 3. The method of claim 1,wherein the infrared thermometer is an ear thermometer having a detectorhead configured to engage an ear canal of the subject and a sensorconfigured to be in proximity to a tympanic membrane of the subject. 4.The method of claim 1, wherein taking successive temperaturemeasurements of the subject comprises: inserting the infraredthermometer into an ear of the subject; waiting until a temperaturethreshold is exceeded; imposing a time delay before beginning a firstear temperature measurement, the time delay selected to allow fullinsertion of the infrared thermometer in the ear; and beginning thefirst ear temperature measurement.
 5. The method of claim 1, wherein themoving time window has a span of 0.5 seconds.
 6. The method of claim 1,wherein successively calculating the average of up to all of theplurality of stored measurements comprises discarding readings from thetime window.
 7. The method of claim 6, wherein the discarded readingsare selected from a group consisting of peak readings, high readings,and low readings.
 8. The method of claim 1, further comprising movingthe time window upon determining that the difference is not less thanthe predetermined value, and wherein successively calculating theaverage of up to all of the plurality of stored measurements isperformed upon determining that the difference is not less than thepredetermined value.
 9. The method of claim 1, wherein outputting thecalculated average temperature is performed upon determining that thedifference is less than the predetermined value.
 10. The method of claim1, wherein the infrared thermometer comprises a thermopile sensor, andthe stored plurality of most recent temperature measurements comprisesoutput values from the thermopile sensor.
 11. The method of claim 1,wherein the stored plurality of most recent temperature measurements inthe moving time window comprises calculated temperatures.
 12. The methodof claim 1, wherein the infrared thermometer comprises: a detector headhousing; a heat sink disposed in the detector head housing, the heatsink having a recess formed therein; a thermopile sensor mounted on theheat sink within the recess, the recess defining an aperture that limitsthe field of view of the thermopile sensor; a thermal sensor; andtemperature determination circuitry configured to calculate atemperature in response to output of the thermopile sensor and thethermal sensor.